A solar thermal power plant based on Direct Steam Central Receiver (DSCR) includes a large field of heliostats and a solar receiver placed on a tower of substantial height. The heliostats focus direct sunlight on to the solar receiver to produce steam to be utilized to run a steam turbine from producing electricity. Typically, the solar thermal power plant operates on a daily cycle, during clear sunlight hours, while shutting down in nights or in cloudy seasons. However, if the solar thermal power plant is to meet increasing electricity demand, it needs to be operable irrespective of the availability of solar light, i.e. in nights or in cloudy seasons. A realization of such a solar thermal power plant generates a requirement of storing solar thermal energy during day times and utilizing thereto in nights or in cloudy seasons. For such requirement, a central receiver including a solar energy storage fluid, such as molten salt, is generally used. The central receiver with molten salt is generally known as Molten Salt Central Receiver (MSCR).
In a typical MSCR system a MSCR, hot and cold storage tanks and a Molten Salt Steam Generator (MSSG) cycle are arranged to utilize the solar energy to produce electricity. In such arrangement, the molten salt fluid heated at the MSCR is stored in the hot storage tank, at temperature of about 565° C., and after thermal energy thereof is being utilized by the MSSG cycle, it is stored in the cold storage tank, at temperature of about 290° C., from where it is further sent to the MSCR to be reheated. The MSSG cycle includes: a steam generator arrangement generally having an economizer, an evaporator and a superheater configured together; a reheat and a multi-stage turbine. The steam generator arrangement utilizes the heat of the hot molten salt and converts feedwater from a feedwater tank in to steam and send it to the multi-stage turbine for the conversion of heat to electricity through a generator. Further, the steam may be reheated in the reheater utilizing the hot molten salt to supply reheated steam for further stage of the multi-stage turbine. Various steam generator technologies may be applied as such for the said purpose. The economizer, evaporator and superheater may be separated in dedicated components or all the three sections may be combined in one single component (known as once-through steam generator). In case of an arrangement with separate components, the evaporator may include one body (often referred to as kettle boiler) or divided into an evaporator and a steam drum for steam separation. Furthermore, each section, like the economizer, evaporator and superheater may be divided into multiple bodies, in series or in parallel.
Irrespective of such varying steam generator technologies, the pressure of steam in the MSSG cycle are generally limited by a so-called pinch limitation in the MSSG cycle, typically at or lower than 115 bars. The pinch limitation in the MSSG is determined by two important factors. Firstly, feedwater temperature shall need to be maintained above a minimum level, typically 240° C., to eliminate the risk of freezing of the molten salt inside of a heat exchanger (economizer, evaporator, superheater (and if included reheat), may be simply be referred to as ‘heat exchange’). Secondly, the temperature of the molten salt leaving the MSSG shall be kept as low as possibly allowed for safe operation of the salt, typically at 290° C. An increase of this outlet temperature decreases the thermal storage capacity, and thus requires additional quantity of salt for the same amount of stored energy. Under these two conditions, the water is heated up in the economizer and starts evaporating at a pressure which is determined by the heat balance of the system, typically at 115 bars or lower. The limitation of the steam pressure resulting from the above mentioned factors has a negative impact on the efficiency of the power plant.